This invention relates generally to optical source and illumination systems. It relates, more particularly, to an apparatus and method for increasing the available optical power from an optical beam from which spatial coherence and/or spatial inhomogeneity are removed. The apparatus and method are particularly useful with an optical beam produced by a laser source.
Lasers and other radiant energy sources have become commonplace in diverse fields, ranging from materials processing to holography to surgery. In many applications, uniform illumination of the target surface is important.
However, the use of coherent energy for illumination presents unique problems in many applications, due at least in part to optical interference effects. The occurrence of constructive and destructive interference of reflected energy, i.e., speckle, and the occurrence of beam intensity inhomogeneity, i.e., hot spots, are examples of these problems.
With further regard to the problem of interference, it is known that objects illuminated with a laser beam or other source of spatially coherent light display a fine grained speckle pattern distributed over the surface. This speckle pattern is invariant with respect to time if the object is immobile. Speckle is believed to be attributable to the finite roughness of the surface being illuminated. For instance, a laser beam, being coherent, reflects off a rough surface at various angles, and the reflections randomly interfere in space with one another and with the incident beam. This constructive and destructive interference appears to the eye or camera as a condition termed "speckle".
In addition to problems manifest in the use of spatially coherent energy, many laser generating systems, e.g., sources, produce output beams having spatially non-homogeneous intensity profiles. One example is laser systems that produce beams having "hot spots", which typically are ring-shaped or star-shaped, and these intensity variations are undesirable in many applications.
Various methods have been used to improve beam uniformity and to reduce speckle during the propagation between a laser source and a target. For instance, known homogenizing apparatus includes kaleidescopes, lens and prism systems, waveguides having substructures to homogenize a laser beam passing therethrough, spatial filters, integrating cavities, and other diffusive surfaces for providing lambertian radiation distribution. See for examples the Buckles U.S. Pat. No. 3,633,999; the Van Ligten et al. U.S. Pat. No. 3,490,827; the Klopotek et al. U.S. Pat. No. 5,109,405; the Grossman U.S. Pat. No. 4,551,628; the Rushworth U.S. Pat. No. 4,309,746; and Burns et al. (1991) Applied Optics 30:2063.
However, the price paid for removing speckle or hot spots from a laser beam is typically the loss of the tight collimation of the beam and the consequent loss of high irradiance. In fact, diffusive mechanisms heretofore used to homogenize optical energy, in general, result in a radiant flux being distributed over too large a solid angle for capture by a practical lens or equivalent element.
It is accordingly an object of this invention to provide an apparatus and method for increasing the power density from diffuse optical energy re-emitted by an optically diffusing surface.
Other objects of the invention will in part be obvious and will in part appear hereinafter.